The use of fossil fuel in gas turbine engines results in the combustion products consisting of carbon dioxide, water vapor, oxides of nitrogen, carbon monoxide, unburned hydrocarbons, oxides of sulfur and particulates. Of these above products, carbon dioxide and water vapor are generally not considered objectionable. In most applications, governmental imposed regulations are further restricting the remainder of the species, mentioned above, emitted in the exhaust gases.
The majority of the products of combustion emitted in the exhaust can be controlled by design modifications, cleanup of exhaust gases and/or regulating the quality of fuel used. For example, particulates in the engine exhaust have been controlled either by design modifications to the combustors and fuel injectors or by removing them by traps and filters. Sulfur oxides are normally controlled by the selection of fuels that are low in total sulfur. This leaves nitrogen oxides and unburned hydrocarbons as the emissions of primary concern in the exhaust gases emitted from the gas turbine engine.
The principal mechanism for the formation of oxides of nitrogen involves the direct oxidation of atmospheric nitrogen and oxygen. The rate of formation of oxides of nitrogen by this mechanism depends mostly upon the flame temperature and to some degree upon the concentration of the reactants. Consequently, a small reduction in flame temperature can result in a large reduction in the nitrogen oxides.
Past and some present systems providing means for reducing the maximum temperature in the combustion zone of a gas turbine combustor have included schemes for introducing more air into the primary combustion zone, recirculating cooled exhaust products into the combustion zone and injecting water spray into the combustion zone. An example of such a system is disclosed in U.S. Pat. No. 4,733,527 issued on Mar. 29, 1988, to Harry A. Kidd. The method and apparatus disclosed therein automatically maintains the NOx emissions at a substantially constant level during all ambient conditions and for no load to full load fuel flows. The water/fuel ratio is calculated for a substantially constant level of NOx emissions at the given operating conditions and, knowing the actual fuel flow to the gas turbine, a signal is generated representing the water metering valve position necessary to inject the proper water flow into the combustor to achieve the desired water/fuel ratio.
Another example of such a water injection system is disclosed in U.S. Pat. No. 4,483,137 issued on Nov. 20, 1984, to Robie L. Faulkner. The patent discloses introducing a liquid coolant into the combustor of the engine. This reduces the flame temperature in the combustor, thereby discouraging the formation of thermal NOx.
In an attempt to reduce NOx emissions without incurring increased operating costs caused by water injection, gas turbine combustion systems have utilized a lean premix approach. In use, experimentation has shown that such lean premix combustion can result in high combustion pressure oscillations and frequent flameouts in the gas turbine load range. The former can reduce the engine and combustion system durability to unacceptable levels while the latter can make the engine unacceptable for part load operation and rapid load reduction.
The above systems used therewith are examples of attempts to reduce the emissions of oxides of nitrogen. The use of water injection increases the operating costs due to the need for supplying water of high purity to the engine. In some applications, a supply of water is difficult to obtain. For example, in desert areas a water supply is basically non-existent, thus, the cost of operation is greatly increased. The operating costs are also increased due to the equipment such as lines, reservoirs and pump.